Yeah, that's a push/pull in your diagram. Not iso.
I've fooled around with a few. They sound great and the boxes are friggin tiny. Nobody does them anymore because of the insane power handling of subs now-a-days. The two subs being out of phase cancel out the odd (or even) ordered harmonic irregularities. That's why it's a popular design for SQ. But they can get loud as shit. Linear Power Blues makes a iso pair as a set you can buy with a kick ass little plexi tube between the two drivers. That setup is like $1k though. Yikes.
Really, the only disadvantage of an isobaric design is that it's a theoretical -3db of the same two subs in a conventional enclosure. Which just means that an iso pair is as loud as a single sub in a conventional. But the fact that you can stuff a pair of 12's in 1 cube and wang the piss out of it and still keep conventional tuning is freaking awesome. Lol.